With almost everything and almost every situation, I'm of the opinion that our interactions with our birds should be positive and voluntary, without fear.
Starting from there, it's a matter of slowly introducing the harness, and backing off when they give signals that they're not comfortable.
I found it a clumsy thing to manipulate at first, but as with everything it becomes more smooth with practice. Even figuring out which way it goes isn't automatic at first ("is this thing upside down and inside out, or both?"). Not only are they learning to accept the harness, we're learning while introducing it. That said, with one of my guys (a Severe Macaw) it's got to the point where he'll put his own head through the centre loop, expects his left wing to be the first through the strap, expects the slack to be pulled to his right wing, lifts his right wing to pull through that loop, knows I'll straighten everything, tighten the buckle, and straighten again, then he lifts his foot because he knows he will step up and we'll go somewhere.
If you've ever seen a parent put clothes on a limp young child (even if they're not resisting, they're also not helping), versus a parent and child who are in a rhythm for the process -- it's akin to that from my perspective.
It took some time because we went slow.
It will also depend on the overall relationship with the bird as a starting point -- if they already trust, then we're pretty far ahead when introducing new behaviours.
I'd say I'm 50% there with my Military, and 0% there with my Conure (who, despite trusting me in other aspects of life, flies away when she sees that very scary spiderweb-looking thing).